This invention relates to the ignition of end burning solid propellant model rocket engines used to propel flying model rockets. The invention can also be applied to the ignition of larger end burning propellant grains in activities not related to model rocketry.
The construction and launching of model rockets has been a popular modeling activity with youngsters and adults for over thirty years. This popularity has been due primarily to the introduction of and consistent reliability of the pre-manufactured model rocket engines. For safety purposes, the propellant contained in these engines is designed to be ignited only with electrically activated igniters, where the electricity is supplied from a remote location, typically a storage battery, connected to the igniter via lead wires.
Because handling safety for these engines is always a concern, particularly with regards to youngsters, igniters sold for use with such engines are commonly designed with low margins of heat output. This igniter design limits danger to the user if he/she attempts to put the igniter to other uses.
Low heat output igniters, while quite reliable if correctly installed in the intended use model rocket engine, are also subject to high unreliability if installed incorrectly. A typical model rocket engine igniter has a pair of plated iron lead wires of approximately two inches in length separated by about one-quarter inch of a fine nickel chromium bridge wire welded to one end of each lead wire. The bridge wire is commonly coated with a relatively low caloric output pyrotechnic mixture, the quantity of which is kept to less than one-tenth gram to limit the heat output and thereby restrict any non-model rocket uses contemplated by the purchaser.
Even though the total caloric heat output of this type of igniter is quite low, it is sufficient to ignite the solid propellant grain within the model rocket engine as long as the igniter is placed in direct contact with the exposed propellant surface. If, however, the igniter is allowed to recede slightly, for example, as little as one-eighth inch, from the surface of the propellant, it is highly probable that the propellant will not be ignited when the igniter is initiated.